Process of casting heavy slips



y 1960 w. A. MAXWELL ETAL 2,944,316 I PROCESS OF CASTING HEAVY SLIPS I Filed Dec. 20, 1956 ML l- 6 i 4 M1 6 Hm T AD 2 n MM 7 5 us w n 0 W WJ o W a 2 x n w u n H a M a W M l W x w. 0 Q A 2 M a u M O 4 Q 4 a 5 i, y a W 4 a ,4 2 1 u w 5 m mired States i 2,944,316- i PROCESS or c'AsriNoHEAvY SL9 s j (Granted under Title 35, Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to the casting of a slip by use. of freezing procedures. i

In the copending case of William A.f-Ma xwell et 21., Serial No. 402,828, tiled January '7, l954,'now Patent No. 2,893,102, granted July 7, :1959, a'pr'ocess of casting metal ceramic and refractory compound powders is described wherein the powders are formed intoa plastic slip and forced into a moldby a plunger}:theiresultingcasting atent hiccbeing frozen and-vacuum dried toelirninate a liquid phase. While this method is satisfactory for many uses there-are limitations to the size of casting possible and the procedure does not lend itself to producing castings-of highsolidity. v V I V In the present invention an importantobject is to provide a method of freeze casting in which use may be out development of a liquid phase. Another object'of importance is to provide a drying step for the frozenicasting which will tend'to maintain the'casting shapethroughout the thawing thereof up to t hetime ofsintering'.

Other objects and many of the attendantadvantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. l is a view in perspective of themold'and the mold filling equipment; a

Fig. 2 is a perspective view of one mold section with the casting therein;

Fig. 3 is a perspective view of the other mold section showing the pin brackets for support in the centrifuge;

Fig. 4.'is a perspective view of a fragment of the cen trifuge for compressing the slip powder; and,

Fig. 5 is a fragmentary view .of a centrifuge unit showing the rotating arm and the slip receptacle mounted for feeding while the unit is rotating. g

Referring to Fig. 1, there is showna horizontal work surface, such as the table 10 to which a vertical backplate ll-is attached., To this backaplate on the "face adjacent the table, .two units, 12 and 13 ,'are attached, unit 12 for n 1ing the vibrator-feed unit and unit 13 the vibrator-mold unit. The vibrator-feed unit is in the form of a rectangular casing 14 supported free of the back-plate 11 by projecting rod elements 15 and containing the usual coil and vibrator armature operated by a 60-cycle current from power lines 16. The vibrator armature is attached to the vibrator support 17 and to this support the powder.

cavity at end 45 of the die plates.

2 t the slip container 18 is attached, as by clamps 19 and 20. The container 1-8 takes the form of a baseboard 2'1 and a receptacle 22 including a funnel cavity 23 tapering to a connecting outlet pipe 24, which may be flexible, for conveyance of the slip to the mold unit 13.

The mold unit 13 similarly, includes a rectangular casing 30 supported outwardly from back-plate 11 byv attachment rods 31 and having a vibrator device supplied with 60-cycle electrical power by power lines 32.

The vibrator plates 33, 34, which are placed over the outer side of casing 30 and attached thereto by clamps 35, are connected to and actuated by the vibrator coil armature, as in the unit 12, and plate 34 supports the mold 36 by means of spaced outwardly extending bolts 37 and 38 joined over the front mold section by bolt clam 39. The mold 36 includes the two mating dies 42 and 43 formed of conventional alloy metals, preferably with low temperature coefiicients of expansion. Except for the mold areas, the dies are identical in size and shape, each die being in the shape of a flat rectangular and'having a recess 48 extending from the mold cavity 44 to the die end 47. When assembled, as by bolts 50 with the mold cavities in registration, the opposed re .cesses 47 .form a passageway 49 for insertionof the powder slip, as shown in Fig. 1, and when assembled as shown in this figure, the slip receptacle outlet pipe 24 projects into this passageway. The mold includes, .also, the base edge plate 52. (Fig. l) which seals the mold Also, as .shown in Fig. 3, pivot straps 53, consisting of flat platesperforated for screw attachment to both dies and having a centrally disposed pivot pin 54, are provided for centrifuge use.

Fig. 4 illustrates diagrammatically a form of centrifuge 59 usable in the compacting step of'the process. This unit includes a vertically positioned power shaft 60, rotatable by any conventional power means, to which an arm 61 is fixed. In form, this arm is a horizontally fiat strip having at its outer end a rectangular recess 62 forming symmetrically positioned terminal support side'members 63. These side members are partially slotted, downwardly from the top-edge and outwardly to form outwardly inclined slots for reception of the mold pivot pins 54. Normally, when the mold is placed in the slots 64 with shaft rotation at zero, it is vertically'pe'ndant, but on shaft rotation themold swings outwardly andif the slip is present this material is compressed to a degree varying with the speed of shaft rotation.

Having indicated the structural elements involved, the details of the process may now be set forth.

The metal is first granulated to powder form with. an average diameter of about nine microns or less and then formed into a slip by mixture of sufiicient water to form a thick'non-zflowing slip. While not essential, it is preferable to include in the mix a binder'so that not only may the amount of water needed and requiring later removal be further reduced but, also, the final mixed prodduct has va consistency and coherence such that, under its own weight, the mix does not slump after prolonged standing but at the same time permits flow of the powder constituent under conditions of vibration or centrifugal pressure of the mixed mass; It is, of course, apparent that the percentage of liquid in the mix varies with the type and particle size of powder, but it may be generally stated that allslips are usable for the described method which contain up to 50 percent less moisture than would be normally required by conventional procedures in powder metallurgy. I h

For example, in the case of molybdenum disilicide powder, 3.55 grams of water is added to 20.0 grams of I a semi-liquid phase, is avoided.

metals.

While the binder used may be selected from numerous flow inhibiting substance, the use of a rubber-base binder has been found particularly successful, the binder including, with rubber latex, an emulsifying agent and water. Slips preparedwith powders having 3 to 5 microns average particle size are effective with 16.5 percent binder. Other'wise stated, 1 to 2 percent of the powder weight in dry rubber makes an adequate binder.

After preparation of the slip it is placed in receptacle 22 and the vibrators in both the receptacle and mold units energized. At once a flow of slip moves into the mold where it is again vibrated to compact and solidify the mass in mold form. The mold is then removed from the vibrator assembly, suspended in the centrifuge 59, and subjected to the centrifugal action of this unit, whereby the slip is closely compacted in the dies. Obviously, high speeds of centrifuge rotation'are desirable, the higher the speed the drier and sounder the resulting castings secured.

After the centrifuge treatment the mold is subjected in appropriate chambers to freezing temperatures, a temperature of 15 F. being found adequate. The casting in the form shown in Fig. 2 at 80, may now be readily handled and, on removal from the mold, it is placed in a container under normal room conditions of temperature and pressure and covered with a drying agent, such as silica gel, activated alumina, fullers earth or bentonite. The mass is then allowed to come to room temperature. As the casting temperature rises, the released moisture is absorbed by the drying agent so that any deformation of the. casting, such as would occtu' on passage through It is notable that the drying agent supports the casting during the drying step by completely enclosing the casting, thus avoiding any local stresses which might distort the original mold configuration. 7

After drying, the casting is sintered at appropriate sintering temperatures, values varying for different metals. For example, for the refractory metal compound molybdenum disilicide a sintering temperature of about 2300 F. is used. If desired, the dried mass may be infiltrated with liquid metals or ceramics.

While the process has been described as applicable to special materials the various process steps are applicable to conventional powder techniques of both ceramics and Also, it should be stated that although spinning the casting in a centrifuge is desirable in obtaining sound products, under some conditions this step may be omitted,

It is noted further that, if desired, the mold may be filled during rotation on the centrifuge, using the so-called splash-filling or other method, the containerll being aligned on arm 70 (Fig. 5) between the mold unit 36 and the arm pivot 71, with conventional retaining means, as hooks 72 holding'the receptacle baseboard 21 in place. The tube 24, being flexible, accommodates itself to the varying position of the pivoted mold, and on rotation a measured or valved mass of the slip is forced into the mold, a valve "73 being indicated.

The factors of paramount importance in the process, as described, include the use of combined gravitational and vibrational forces to fill the mold with a low-liquid content slip, whereby the particled mass is solidly packed without being unduly forced as by extrusion methods. Additionally, the rubber-based binder, as employed, imparts a texture to the slip which is most effective in insuring adequate flow under vibrational treatment. Other important factors include the firm compaction of the slip in centrifuge treatment and the successful maintenance of casting form during drying by use of a combined packing and drying agent.

Obviously many modifications and variations of the 4 present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

-1. A process of making articles from powdered materials which comprises mixing the powder with a liquid in such amount as to form a thick slip which flows only under pressure, applying pressure to said slip to obtain a free slip flow into a mold to form a mold mass, subjecting the mold mass to centrifugal forces to compact the mass powder in the mold, freezing the mold mass to form a casting, drying the casting by heating it to standard room temperature at normal atmospheric pressure and by application of a solid moisture absorbing agent to the casting surface, and sintering said casting.

2. A process of making articles from powdered materials which comprises mixing the powder with a liquid in such amount as to form a thick slip which flows only under pressure applying pressure to said slip to obtain a free slip flow into a mold under both gravitational and vibrational forces to form a mold mass, subjecting the mold mass to centrifugal forces to compact the mass powder in the mold, freezing the mold mass to ,form a casting, drying the casting by heating it to standard room temperature at normal atmospheric pressure and by application of a solid moisture absorbing agent to the casting .surface, and sintering said casting.

3. A process of making articles from powdered materials which comprises mixingthe powder'with a liquidin such amount as to form a thick, non-flowing slip, applying pressure to force said slip. into a mold to form a mold mass, vibrating said mold mass to solidify the same, sub- .jecting the mold mass to centrifugal forces to compact the mass powder in the mold, freezing the mold mass to form a casting, drying the casting by heating it to standmass, subjecting the, mold mass to centrifugal forces to compact the mass powder, in the mold, freezing the mold mass to form a casting, drying the casting by he'ating. it to standard room temperature at normal atmospheric pressure and by application of a solid moisture. absorb .ing agent to the casting surface, and sintering'said casting.

References Cited'in the file of this patent UNITED STATES PATENTS 1,347,484 Brown July"20, 1920 1,514,926 Powers Nov. 11, 1924 1,612,916 Gorton Jan. 4, 1927 1,656,387 Nelson Ian. 17, 1928 1,680,832 Wilder Aug. 14, 1928 1,694,563 Ross et a1. Dec. 11, 1928 1,768,546 Curtis July 1, 1930 2,026,624 Flower Jan. 7, 1936 1 2,048,253 Freyssinet July 21, 1936 2,397,897 'Wcnger Apr. 2, 1946 2,438,817 Nalick Mar. 30, 1948 2,542,757 Dubin Feb. 20, 1951 2,765,512 Nesbit Oct. 9, 1956 2,893,102 Maxwell et al. July 7, 1959 FOREIGN PATENTS 70,734 1915 Germany May 15, 

